private $my_variable;
$my_variable = Null;

if (isset($my_variable))		//would be better named isnotnull()
	$my_variable=1;

if (is_null($my_variable))
	$my_variable=1;

Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.